Openness

The amount of water and air that can quickly pass
through a net will depend on the openness (open area / total area)
of the material.
Openness should be the first consideration in choosing a netting material as
it effects the seaworthiness of the boat. With catamarans, the forward
nets will get pounded from underneath by water forces. With trimarans
the windward bow net will catch wind pressure while the leeward net
will catch the water.

Generally, this is more of a concern for offshore sailing where heavy
weather may be encountered. Boats with larger net areas require greater
openness than those with smaller net areas. High preformance boats
require greater openness than cruising boats. Nets closer to the ends
of the boat need to be more open than nets near the center. The following
chart shows the openness percentage for several of the different
materials on a typical sized net.

Openness Precentage
for a 50 sq ft net area with 3" lacing gaps

Polypropylene Mesh

SquareHole Mesh

Coated Polyester Mesh

1-1/2" Ultra Pro

1" Webbing Net w/2" Avg Gaps

1" Webbing Net w/1-1/2" Avg Gaps

2" 3Ply Webbing Net w/2-1/2" Avg Gaps

The following table lists some very general guidelines for the minimum
net openness for offshore sailing. This is a guideline only, based
on typical cruising multihulls, and individual boat requirements
may vary considerably from this.

Boat type, and net size
(total size, port and starboard)

Minimum net openness

Catamaran fwd nets under 50 sq ft

30%

Catamaran fwd nets 50 to 120 sq ft

40%

Catamaran fwd nets over 120 sq ft

50%

preformance cruising Trimaran bow nets

70%

cruising Trimaran bow nets under 100 sq ft

50%

Trimaran wing nets under 60 sq ft

10%

Trimaran wing nets 60 to 120 sq ft

30%

Trimaran wing nets over 120 sq ft

50%

Openness figures for all netting materials. calculated for a 50
sq ft opening including borders and lacing gaps